Ion beams can be produced by many different types of ion sources. Initially, ion beams proved useful in physics research. A notable early example use of an ion source was in the first vacuum mass spectrometer invented by Aston and used to identify elemental isotopes. Ions were extracted from an ion source in which a vacuum arc was formed between two metal electrodes.
Since those early days, ion beams have found application in a variety of industrial applications, most notably, as a technique for introducing dopants into a silicon wafer. While a number of ion sources have been developed for different purposes, the physical methods by which ions can be created are, however, quite limited and, with the exception of a few ion sources exploiting such phenomena as direct sputtering or field emission from a solid or liquid, are restricted to the extraction of ions from an arc or plasma.
The plasma in an ion source is generated by a low-pressure discharge between electrodes, one of which is often a cathode of electron-emitting filaments, excited by direct current, pulsed, or high-frequency fields. An ion implantation apparatus having an ion source utilizing electron emitting filaments as a cathode is disclosed in U.S. Pat. No. 4,714,834 to Shubaly, which is incorporated herein in its entirety by reference. The plasma formed in this way is usually enhanced by shaped static magnetic fields. The active electrodes, particularly the hot filament cathode and the plasma chamber walls which function as the anode are attacked by energetic and chemically active ions and electrons. The lifetime of the ion source is often limited to a few hours by these interactions, especially if the gaseous species introduced into the ion source to form the plasma are in themselves highly reactive, e.g., phosphorous, fluorine, boron, etc.
The increasing use of ion beams in industry (e.g., ion implantation, ion milling and etching) has placed a premium on the development of ion sources having a longer operational life. Compared to filament ion sources, microwave-energized ion sources operate at lower ionization gas pressure in the plasma chamber resulting in higher electron temperatures (eV), a desirable property. However, prior art microwave energy ion sources proved, like the filament ion sources, to have limited operational lives (about two hours) before repair/replacement was required.
U.S. Pat. No. 4,883,968 to Hipple et al., which is incorporated herein in its entirety by reference, discloses one such microwave energized ion source. The Hipple et al. ion source includes a window bounding one end of a cylindrical stainless steel plasma chamber. The window functions as both a microwave energy interface region and a pressure or vacuum seal. As a microwave energy interface region, the window transmits microwave energy from a microwave waveguide to source materials within the plasma chamber. As a vacuum seal, the window provides a pressure seal between the plasma chamber, which is evacuated, and the unevacuated regions of the ion source, e.g., the region through which the waveguide extends. The Hipple et al. window is comprised of a sandwiched, parallel arrangement of three dielectric disks (two being made of boron nitride and the third being alumina) and one quartz disk. A thin boron nitride disk bounds the plasma chamber. Adjacent the thin boron nitride disk is a thicker boron nitride disk followed in order by the alumina disk and finally the quartz disk.
The boron nitride disks exhibit a high melting point and good thermal conductivity. Microwave energy is delivered to the window by a waveguide which extends from a microwave source to a flange adjacent the window's quartz disk. The flange has a central rectangular opening through which microwave energy passes from the waveguide to the window. The quartz disk functions as a vacuum seal to maintain the vacuum drawn in the plasma chamber. The alumina plate serves as an impedance matching plate to tune the microwave energy. Impedance matching is required to minimize undesirable microwave energy reflection by the plasma chamber plasma. While the Hippie et al. ion source represents an improvement over prior art ion sources in terms of a number of operating characteristics including longevity, designing an ion source having a longer operational life continues to be a goal of manufacturers of ion implantation systems.
The microwave window is necessarily exposed to high temperatures present in the plasma chamber (&lt;800.degree. C.). Moreover, the microwave energy interface region must be hot to remain clean and provide acceptable microwave energy coupling between the microwave waveguide and the plasma in the plasma chamber when ionizing source materials which include condensable species such as phosphorous. However, it has been found that the vacuum seal has an increased operating life when it is not subjected to extreme heat or chemical attack from the energized ions and electrons in the plasma.
A hollow tube waveguide was conventionally used in prior art devices to feed microwave energy from the microwave generator to the plasma chamber. The waveguide mode of microwave energy transmission is limited to a range of frequencies. If the generated microwave frequency is outside the range, the waveguide will not transmit the microwave energy, a cut-off condition will result. Transmission frequency range limitations are a disadvantage of the waveguide microwave energy transmission mode.